An endotoxin is a toxic substance present in the bacterial body component of a bacteria, and released when a bacteria is killed. A structural component of endotoxin is lipopolysaccharide, which is produced inside the bacterial body by a bacteria living during the production process of a medicinal drug product, and contaminates the medicinal drug product when it is killed. Adsorbents by activated charcoal or ion-exchanger, filtration methods using membranes, membrane filters or the like, decomposition methods using high temperature/high pressure treatments or acid or alkali, are known methods for removing endotoxin. All the methods have merits and demerits, posing problems for industrial use. For instance, removal of endotoxin in medicinal drug product manufacturing sometimes cannot be carried out under harsh conditions from the point of maintaining the stability of the original medicinal drug product, or due to the tiny amounts of endotoxin present, although adsorption is carried out successfully in a laboratory, the adsorption cannot be carried out satisfactorily in an actual industrial scale, or the medicinal drug product itself is adsorbed onto the adsorbent, such that it is by no means satisfactory. In contrast, a number of adsorbents have been disclosed in recent years, in order to allow endotoxin adsorption to be also satisfactory for the industry. For instance, an adsorbent constituted from a poly-amino acid containing a modified group having a fatty series group and/or an aryl at the extremity of a side chain and/or a main chain has been disclosed (Japanese Patent Publication No. H6-16843), and an adsorbent comprising a poly-amino acid spherical particle serving as a carrier and a imidazole derivative conjugated thereto has been disclosed (Japanese Patent Application Laid-open No. H1-127039). In addition, a method whereby a basic substance is immobilized and the pore size of the adsorbent is controlled to adsorb endotoxin selectively has been disclosed (Japanese Patent Application Laid-open No. 2002-263486).